Retrospective snapshots in log structured storage systems

ABSTRACT

One embodiment provides a method for retrospective snapshot creation including creating, by a processor, a first snapshot that captures logical state of a data store at a first time in a time range. Creation of the first snapshot is based on determining existence of a second snapshot that captures logical state of the data store and recording a retrospective snapshot at a last valid log address offset prior to the first time upon a determination that the second snapshot exists based on determining at least one of: whether log address offsets from a first log entry of a log to a log entry of the log at the first time are contiguous and whether log address offsets from the second snapshot to the first time are contiguous.

BACKGROUND

Typical log-structured storage systems, stores record data in temporalorder in a “log.” These typical systems allow basic primitiveoperations, such as insert, update, delete, read. Each update of dataresults in a new record being inserted at the tail of the “log.” Eachdelete results in a tombstone object being inserted at the tail of thelog. Additionally, background garbage collection process compacts thedata reclaiming space that does not contain valid data.

SUMMARY

Embodiments relate to retrospective snapshot creation for log-structuredstorage systems. One embodiment provides a method for retrospectivesnapshot creation including creating, by a processor, a first snapshotthat captures logical state of a data store at a first time in a timerange. Creation of the first snapshot is based on determining existenceof a second snapshot that captures logical state of the data store andrecording a retrospective snapshot at a last valid log address offsetprior to the first time upon a determination that the second snapshotexists based on determining at least one of: whether log address offsetsfrom a first log entry of a log to a log entry of the log at the firsttime are contiguous and whether log address offsets from the secondsnapshot to the first time are contiguous.

These and other features, aspects and advantages of the presentinvention will become understood with reference to the followingdescription, appended claims and accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a cloud computing environment, according to anembodiment;

FIG. 2 depicts a set of abstraction model layers, according to anembodiment;

FIG. 3 is a network architecture for retrospective snapshots inlog-structured storage systems, according to an embodiment;

FIG. 4 shows a representative hardware environment that may beassociated with the servers and/or clients of FIG. 1, according to anembodiment;

FIG. 5 is a block diagram illustrating a processor for retrospectivesnapshots in log-structured storage systems, according to an embodiment;

FIG. 6 illustrates interaction of garbage collection with snapshots;

FIG. 7 is a block diagram illustrating flow for timestamp basedretrospective snapshots, according to an embodiment; and

FIG. 8 illustrates a block diagram for a process for retrospectivesnapshots in log-structured storage systems, according to oneembodiment.

DETAILED DESCRIPTION

The descriptions of the various embodiments have been presented forpurposes of illustration, but are not intended to be exhaustive orlimited to the embodiments disclosed. Many modifications and variationswill be apparent to those of ordinary skill in the art without departingfrom the scope and spirit of the described embodiments. The terminologyused herein was chosen to best explain the principles of theembodiments, the practical application or technical improvement overtechnologies found in the marketplace, or to enable others of ordinaryskill in the art to understand the embodiments disclosed herein.

It is understood in advance that although this disclosure includes adetailed description of cloud computing, implementation of the teachingsrecited herein are not limited to a cloud computing environment. Rather,embodiments of the present invention are capable of being implemented inconjunction with any other type of computing environment now known orlater developed.

One or more embodiments provide for retrospective snapshot creation. Oneembodiment includes creating, by a processor, a first snapshot thatcaptures logical state of a data store at a first time in a time range.Creation of the first snapshot is based on determining existence of asecond snapshot that captures logical state of the data store andrecording a retrospective snapshot at a last valid log address offsetprior to the first time upon a determination that the second snapshotexists based on determining at least one of: whether log address offsetsfrom a first log entry of a log to a log entry of the log at the firsttime are contiguous and whether log address offsets from the secondsnapshot to the first time are contiguous.

Cloud computing is a model of service delivery for enabling convenient,on-demand network access to a shared pool of configurable computingresources (e.g., networks, network bandwidth, servers, processing,memory, storage, applications, virtual machines (VMs), and services)that can be rapidly provisioned and released with minimal managementeffort or interaction with a provider of the service. This cloud modelmay include at least five characteristics, at least three servicemodels, and at least four deployment models.

Characteristics are as follows:

On-demand self-service: a cloud consumer can unilaterally provisioncomputing capabilities, such as server time and network storage, asneeded and automatically, without requiring human interaction with theservice's provider.

Broad network access: capabilities are available over a network andaccessed through standard mechanisms that promote use by heterogeneous,thin or thick client platforms (e.g., mobile phones, laptops, and PDAs).

Resource pooling: the provider's computing resources are pooled to servemultiple consumers using a multi-tenant model, with different physicaland virtual resources dynamically assigned and reassigned according todemand. There is a sense of location independence in that the consumergenerally has no control or knowledge over the exact location of theprovided resources but may be able to specify location at a higher levelof abstraction (e.g., country, state, or data center).

Rapid elasticity: capabilities can be rapidly and elasticallyprovisioned and, in some cases, automatically, to quickly scale out andrapidly released to quickly scale in. To the consumer, the capabilitiesavailable for provisioning often appear to be unlimited and can bepurchased in any quantity at any time.

Measured service: cloud systems automatically control and optimizeresource use by leveraging a metering capability at some level ofabstraction appropriate to the type of service (e.g., storage,processing, bandwidth, and active consumer accounts). Resource usage canbe monitored, controlled, and reported, thereby providing transparencyfor both the provider and consumer of the utilized service.

Service Models are as follows:

Software as a Service (SaaS): the capability provided to the consumer isthe ability to use the provider's applications running on a cloudinfrastructure. The applications are accessible from various clientdevices through a thin client interface, such as a web browser (e.g.,web-based email). The consumer does not manage or control the underlyingcloud infrastructure including network, servers, operating systems,storage, or even individual application capabilities, with the possibleexception of limited consumer-specific application configurationsettings.

Platform as a Service (PaaS): the capability provided to the consumer isthe ability to deploy onto the cloud infrastructure consumer-created oracquired applications created using programming languages and toolssupported by the provider. The consumer does not manage or control theunderlying cloud infrastructure including networks, servers, operatingsystems, or storage, but has control over the deployed applications andpossibly application-hosting environment configurations.

Infrastructure as a Service (IaaS): the capability provided to theconsumer is the ability to provision processing, storage, networks, andother fundamental computing resources where the consumer is able todeploy and run arbitrary software, which can include operating systemsand applications. The consumer does not manage or control the underlyingcloud infrastructure but has control over operating systems, storage,deployed applications, and possibly limited control of select networkingcomponents (e.g., host firewalls).

Deployment Models are as follows:

Private cloud: the cloud infrastructure is operated solely for anorganization. It may be managed by the organization or a third party andmay exist on-premises or off-premises.

Community cloud: the cloud infrastructure is shared by severalorganizations and supports a specific community that has shared concerns(e.g., mission, security requirements, policy, and complianceconsiderations). It may be managed by the organizations or a third partyand may exist on-premises or off-premises.

Public cloud: the cloud infrastructure is made available to the generalpublic or a large industry group and is owned by an organization sellingcloud services.

Hybrid cloud: the cloud infrastructure is a composition of two or moreclouds (private, community, or public) that remain unique entities butare bound together by standardized or proprietary technology thatenables data and application portability (e.g., cloud bursting for loadbalancing between clouds).

A cloud computing environment is a service oriented with a focus onstatelessness, low coupling, modularity, and semantic interoperability.At the heart of cloud computing is an infrastructure comprising anetwork of interconnected nodes.

Referring now to FIG. 1, an illustrative cloud computing environment 50is depicted. As shown, cloud computing environment 50 comprises one ormore cloud computing nodes 10 with which local computing devices used bycloud consumers, such as, for example, personal digital assistant (PDA)or cellular telephone 54A, desktop computer 54B, laptop computer 54C,and/or automobile computer system 54N may communicate. Nodes 10 maycommunicate with one another. They may be grouped (not shown) physicallyor virtually, in one or more networks, such as private, community,public, or hybrid clouds as described hereinabove, or a combinationthereof. This allows the cloud computing environment 50 to offerinfrastructure, platforms, and/or software as services for which a cloudconsumer does not need to maintain resources on a local computingdevice. It is understood that the types of computing devices 54A-N shownin FIG. 2 are intended to be illustrative only and that computing nodes10 and cloud computing environment 50 can communicate with any type ofcomputerized device over any type of network and/or network addressableconnection (e.g., using a web browser).

Referring now to FIG. 2, a set of functional abstraction layers providedby the cloud computing environment 50 (FIG. 1) is shown. It should beunderstood in advance that the components, layers, and functions shownin FIG. 2 are intended to be illustrative only and embodiments of theinvention are not limited thereto. As depicted, the following layers andcorresponding functions are provided:

Hardware and software layer 60 includes hardware and softwarecomponents. Examples of hardware components include: mainframes 61; RISC(Reduced Instruction Set Computer) architecture based servers 62;servers 63; blade servers 64; storage devices 65; and networks andnetworking components 66. In some embodiments, software componentsinclude network application server software 67 and database software 68.

Virtualization layer 70 provides an abstraction layer from which thefollowing examples of virtual entities may be provided: virtual servers71; virtual storage 72; virtual networks 73, including virtual privatenetworks; virtual applications and operating systems 74; and virtualclients 75.

In one example, a management layer 80 may provide the functionsdescribed below. Resource provisioning 81 provides dynamic procurementof computing resources and other resources that are utilized to performtasks within the cloud computing environment. Metering and pricing 82provide cost tracking as resources are utilized within the cloudcomputing environment and billing or invoicing for consumption of theseresources. In one example, these resources may comprise applicationsoftware licenses. Security provides identity verification for cloudconsumers and tasks as well as protection for data and other resources.User portal 83 provides access to the cloud computing environment forconsumers and system administrators. Service level management 84provides cloud computing resource allocation and management such thatrequired service levels are met. Service Level Agreement (SLA) planningand fulfillment 85 provide pre-arrangement for, and procurement of,cloud computing resources for which a future requirement is anticipatedin accordance with an SLA.

Workloads layer 90 provides examples of functionality for which thecloud computing environment may be utilized. Examples of workloads andfunctions which may be provided from this layer include: mapping andnavigation 91; software development and lifecycle management 92; virtualclassroom education delivery 93; data analytics processing 94; andtransaction processing 95. As mentioned above, all of the foregoingexamples described with respect to FIG. 2 are illustrative only, and theinvention is not limited to these examples.

It is understood all functions of one or more embodiments as describedherein may be typically performed by the processing system 300 (FIG. 3)or the autonomous cloud environment 410 (FIG. 4), which can be tangiblyembodied as hardware processors and with modules of program code.However, this need not be the case for non-real-time processing. Rather,for non-real-time processing the functionality recited herein could becarried out/implemented and/or enabled by any of the layers 60, 70, 80and 90 shown in FIG. 2.

It is reiterated that although this disclosure includes a detaileddescription on cloud computing, implementation of the teachings recitedherein are not limited to a cloud computing environment. Rather, theembodiments of the present invention may be implemented with any type ofclustered computing environment now known or later developed.

FIG. 3 illustrates a network architecture 300, in accordance with oneembodiment. As shown in FIG. 3, a plurality of remote networks 302 areprovided, including a first remote network 304 and a second remotenetwork 306. A gateway 301 may be coupled between the remote networks302 and a proximate network 308. In the context of the present networkarchitecture 300, the networks 304, 306 may each take any formincluding, but not limited to, a LAN, a WAN, such as the Internet,public switched telephone network (PSTN), internal telephone network,etc.

In use, the gateway 301 serves as an entrance point from the remotenetworks 302 to the proximate network 308. As such, the gateway 301 mayfunction as a router, which is capable of directing a given packet ofdata that arrives at the gateway 301, and a switch, which furnishes theactual path in and out of the gateway 301 for a given packet.

Further included is at least one data server 314 coupled to theproximate network 308, which is accessible from the remote networks 302via the gateway 301. It should be noted that the data server(s) 314 mayinclude any type of computing device/groupware. Coupled to each dataserver 314 is a plurality of user devices 316. Such user devices 316 mayinclude a desktop computer, laptop computer, handheld computer, printer,and/or any other type of logic-containing device. It should be notedthat a user device 311 may also be directly coupled to any of thenetworks in some embodiments.

A peripheral 320 or series of peripherals 320, e.g., facsimile machines,printers, scanners, hard disk drives, networked and/or local storageunits or systems, etc., may be coupled to one or more of the networks304, 306, 308. It should be noted that databases and/or additionalcomponents may be utilized with, or integrated into, any type of networkelement coupled to the networks 304, 306, 308. In the context of thepresent description, a network element may refer to any component of anetwork.

According to some approaches, methods and systems described herein maybe implemented with and/or on virtual systems and/or systems, whichemulate one or more other systems, such as a UNIX system that emulatesan IBM z/OS environment, a UNIX system that virtually hosts a MICROSOFTWINDOWS environment, a MICROSOFT WINDOWS system that emulates an IBMz/OS environment, etc. This virtualization and/or emulation may beimplemented through the use of VMWARE software in some embodiments.

FIG. 4 shows a representative hardware system 400 environment associatedwith a user device 416 and/or server 314 of FIG. 3, in accordance withone embodiment. In one example, a hardware configuration includes aworkstation having a central processing unit 410, such as amicroprocessor, and a number of other units interconnected via a systembus 412. The workstation shown in FIG. 4 may include a Random AccessMemory (RAM) 414, Read Only Memory (ROM) 416, an I/O adapter 418 forconnecting peripheral devices, such as disk storage units 420 to the bus412, a user interface adapter 422 for connecting a keyboard 424, a mouse426, a speaker 428, a microphone 432, and/or other user interfacedevices, such as a touch screen, a digital camera (not shown), etc., tothe bus 412, communication adapter 434 for connecting the workstation toa communication network 435 (e.g., a data processing network) and adisplay adapter 436 for connecting the bus 412 to a display device 438.

In one example, the workstation may have resident thereon an operatingsystem, such as the MICROSOFT WINDOWS Operating System (OS), a MAC OS, aUNIX OS, etc. In one embodiment, the system 400 employs a POSIX® basedfile system. It will be appreciated that other examples may also beimplemented on platforms and operating systems other than thosementioned. Such other examples may include operating systems writtenusing JAVA, XML, C, and/or C++ language, or other programming languages,along with an object oriented programming methodology. Object orientedprogramming (OOP), which has become increasingly used to develop complexapplications, may also be used.

FIG. 5 is a block diagram illustrating a processing node 500 forretrospective snapshots in log-structured storage systems, according toan embodiment. The processing node 500 includes one or more processors510, a snapshot interface 530 and a memory 520. In one embodiment, eachprocessor(s) 510 performs processing of snapshots in log structures. Thesnapshot interface 530 provides reconstruction of a consistent logicalstate of a store, where a log is replayed from the beginning of the log(or from a valid checkpoint) until a snapshot offset. The snapshotinterface also provides for limiting garbage collection to preventgarbage collection across snapshot boundaries. The term garbagecollection refers to reclaiming “disk space” occupied by stale entriesin the log. For example, when a record is inserted, an entry is added tothe tail of the log. When the same record is deleted, a tombstone entryis added to the tail of the log. The tombstone entry refers to theoriginal location of the data on disk as created by the insert. The diskspace occupied by the original inserted record may be garbage collected(provided the system is not maintaining older versions). Stale data maybe the result of records that have been deleted or updated. Updatesresult in stale data because older versions of the data that aremaintained in the log are no longer needed. Note that in alog-structured store, every insert, update or delete operation resultsin a record being inserted at the tail of the log.

In one embodiment, snapshots in a log structure store capture thelogical state of a data store as of a given point in time. Snapshots ina log-structured store in essence record the last offset (“snapshotoffset”) in the “log” at a given point in time. To reconstruct theconsistent logical state of the store, the log is replayed from thebeginning of the log (or from a valid checkpoint) until the “snapshotoffset” and the in-memory structures (such as indexes) are reconstructedto effectively reconstruct a snapshot of the store.

FIG. 6 illustrates interaction of garbage collection (GC) withsnapshots. In order to achieve logical correctness, certain garbagecollection should not be performed across snapshot boundaries. Forinstance, a delete may have occurred after a snapshot point. Thisimplies the data existed as of the time of the snapshot. Garbagecollecting the data record based on the aforementioned delete wouldresult in an inconsistency. The following rules are applied for thesnapshots and GC zones. Rule 1: GC scope is limited to a GC Zone, whichmaintains snapshot fidelity. Rule 2: Tombstones that refer to objectsacross snapshot boundaries are known as Cross-Snapshot Tombstones (CSTs)and cannot be garbage collected as long as the snapshots are valid. Rule3: Tombstones which refer to objects within a GC Zone are referred to asIntra-snapshot tombstones (ISTs) and can be garbage collected. Garbagecollecting within a GC Zone: 1. Objects eligible for garbage collectionare described by the rules above. 2. Index Checkpoints which lie withina snapshot boundary and live indexes may be impacted by objectrelocation during garbage collection.

Some of the issues related to the interaction of GC with snapshots arethat consistent snapshots cannot be taken in retrospect. For instance,the following requests cannot be made: “Create a snapshot as of lastweek,” and “Create a snapshot as of last month.” The reason for notbeing able to make the aforementioned requests is because the GC processmay have already interfered with the logical correctness. Consider thefollowing sequence of events:

-   -   Insert record with key K1 in week 1.    -   Delete record with key K1 in week 2.    -   In week 3, since no snapshot exists yet, GC removes both the        insert and delete records with key K1.    -   In week 4, the following request is issued “create snapshot as        of week 1.” This operation is unsuccessful since the record with        key K1 has already been deleted and would leave the snapshot        inconsistent. However, this rule is overly restrictive. For        instance, if the workload consisted mostly of inserts (such as        in a time series, or Internet of Things (IoT) or sensor        scenario), very little ad-hoc GC occurs.

To overcome the shortcomings and issues mentioned above, one or moreembodiments provide processing for creation of consistent snapshots inretrospective when the following conditions are met. If all “log”addresses from the last snapshot (or the beginning of the log, if nosnapshot exists previously) are present contiguously in the log, providefor creation of a snapshot in retrospective. The following underlyingguarantee is required from the system: log addresses should never bereused and should always be monotonically increasing and contiguous.This is true since upper layer indexes are of the form <key, logaddress > and reuse of log addresses will result in corruption. As longas the above guarantee is met, irrespective of whether the log is alwaysmaintained in chronological order or not, the retrospective snapshotcreation will hold.

In one embodiment, retrospective snapshot creation is based on aspecific timestamp or a specific log address. In one embodiment,retrospective_snapshot S_(x) is created for time T_(x). In one example,a looser definition could be creation of a snapshot providing a timerange, allowing the system to choose any time T_(x) which meets thecriterion. For example, by providing a time range (sometime betweenMonday and Tuesday). Then the system creates retrospective_snapshotS_(x) for the highest time in that range that satisfies the conditionsdescribed previously. In one embodiment, internally both operations aretranslated to a retrospective_snapshot for the log address correspondingto the timestamp T_(x). If the conditions described above are not met,the snapshot operation cannot proceed and will fail.

In one embodiment, in order to support retrospective snapshot by time, atimestamp should be recorded for each entry in the log. A secondaryindex on the timestamp may be used to quickly locate the log address forthe corresponding key. In one embodiment, a snapshot is associated witha specific log address in the log.

FIG. 7 is a block diagram illustrating flow for a process 700 fortimestamp based retrospective snapshots, according to an embodiment.Process 700 describes the process of creating a retrospective snapshotgiven a timestamp. Process 700 provides for trade-off of spaceefficiency with need to create snapshots in retrospective. In block 710a request to create a snapshot operation is initiated at time T_(n) fortime T_((n-k)) is received, where n and k are positive integers. Inblock 720 process 700 finds the log offset (L_(k)) corresponding to thesmallest timesamp T_(x)>T_((n-k)) where k and x are positive integers.In block 730, it is determined if the latest snapshot S_(x) (i.e., asnapshot with the highest log offset) prior to T_(x) exists (i.e. asnapshot S_(x) recorded in the log before time T_(x)). If it isdetermined that the latest snapshot S_(x) prior to T_(x) does exist,process 700 proceeds to block 750. Otherwise, process 700 proceeds toblock 740.

In one embodiment, in block 740 it is determined whether all log offsetsfrom the beginning of the log to L_(k) are contiguous or not. If it isdetermined that all log offsets from the beginning of the log to L_(k)are contiguous (implying that no garbage collection that will affect thesnapshot fidelity has occurred), process 700 proceeds to block 770.Otherwise process 700 proceeds to block 760. In one embodiment, in block750 it is determined whether all log offsets from S_(x) to L_(k) arecontiguous. If it is determined that all log offsets from S_(x) to L_(k)are contiguous, process 700 proceeds to block 770. Otherwise process 700proceeds to block 760.

In one embodiment, in block 760, since it is determined that all logoffsets from the beginning of the log to T_(x) are not contiguous, thesnapshot attempt failed and process 700 ends. In block 770 the snapshotS_(X+1) location at the last valid offset prior to T_(x) is recorded.After block 770, process 700 proceeds to block 780 where the snapshotattempt is noted as being successful; then process 700 ends.

FIG. 8 illustrates a block diagram for a process 800 for retrospectivesnapshots in log-structured storage systems, according to oneembodiment. In block 810 process 800 includes creating (e.g., by aprocessor 510), a first snapshot that captures logical state of a datastore at a first time in a time range. In block 820 creation of thefirst snapshot is based on: determining existence of a second snapshot(i.e., older snapshot) that captures logical state of the data store. Inone embodiment, the second snapshot is not a requirement and is only anoptimization that allows for continuity checks to be performed from thelatest snapshot to a new desired snapshot offset and not form thebeginning of the log each time. Block 830 includes recording aretrospective snapshot at a last valid log address offset prior to thefirst time upon a determination that the second snapshot exists based onblock 840 determining at least one of: whether log address offsets froma first log entry of a log to a log entry of the log at the first timeare contiguous, and whether log address offsets from the second snapshotto the first time are contiguous.

In one embodiment, process 800 provides that log addresses are notreused, monotonically increase and are contiguous. In one example, inprocess 800 the second snapshot has a largest log address offset thanother snapshots prior to the first time, and the time range is between afirst date and a second date. In one embodiment, the second snapshot isa retrospective snapshot for a log address corresponding to a timestampat the first time.

Process 800 may further include that creation of the retrospectivesnapshot fails upon at least one of: the log address offsets from thefirst log entry to the log entry at the first time are non-contiguous,and the log address offsets from the second snapshot to the first timeare non-contiguous. Process 800 may additionally include recording atimestamp for each log entry in the log. Process 800 may further includelocating a log address for a corresponding key using a secondary indexon a timestamp.

As will be appreciated by one skilled in the art, aspects of the presentinvention may be embodied as a system, method or computer programproduct. Accordingly, aspects of the present invention may take the formof an entirely hardware embodiment, an entirely software embodiment(including firmware, resident software, micro-code, etc.) or anembodiment combining software and hardware aspects that may allgenerally be referred to herein as a “circuit,” “module” or “system.”Furthermore, aspects of the present invention may take the form of acomputer program product embodied in one or more computer readablemedium(s) having computer readable program code embodied thereon.

Any combination of one or more computer readable medium(s) may beutilized. The computer readable medium may be a computer readable signalmedium or a computer readable storage medium. A computer readablestorage medium may be, for example, but not limited to, an electronic,magnetic, optical, electromagnetic, infrared, or semiconductor system,apparatus, or device, or any suitable combination of the foregoing. Morespecific examples (a non-exhaustive list) of the computer readablestorage medium would include the following: an electrical connectionhaving one or more wires, a portable computer diskette, a hard disk, arandom access memory (RAM), a read-only memory (ROM), an erasableprogrammable read-only memory (EPROM or Flash memory), an optical fiber,a portable compact disc read-only memory (CD-ROM), an optical storagedevice, a magnetic storage device, or any suitable combination of theforegoing. In the context of this document, a computer readable storagemedium may be any tangible medium that can contain, or store a programfor use by or in connection with an instruction execution system,apparatus, or device.

A computer readable signal medium may include a propagated data signalwith computer readable program code embodied therein, for example, inbaseband or as part of a carrier wave. Such a propagated signal may takeany of a variety of forms, including, but not limited to,electro-magnetic, optical, or any suitable combination thereof. Acomputer readable signal medium may be any computer readable medium thatis not a computer readable storage medium and that can communicate,propagate, or transport a program for use by or in connection with aninstruction execution system, apparatus, or device.

Program code embodied on a computer readable medium may be transmittedusing any appropriate medium, including but not limited to wireless,wireline, optical fiber cable, RF, etc., or any suitable combination ofthe foregoing.

Computer program code for carrying out operations for aspects of thepresent invention may be written in any combination of one or moreprogramming languages, including an object oriented programming languagesuch as Java, Smalltalk, C++ or the like and conventional proceduralprogramming languages, such as the “C” programming language or similarprogramming languages. The program code may execute entirely on theuser's computer, partly on the user's computer, as a stand-alonesoftware package, partly on the user's computer and partly on a remotecomputer or entirely on the remote computer or server. In the latterscenario, the remote computer may be connected to the user's computerthrough any type of network, including a local area network (LAN) or awide area network (WAN), or the connection may be made to an externalcomputer (for example, through the Internet using an Internet ServiceProvider).

Aspects of the present invention are described below with reference toflowchart illustrations and/or block diagrams of methods, apparatus(systems) and computer program products according to embodiments of theinvention. It will be understood that each block of the flowchartillustrations and/or block diagrams, and combinations of blocks in theflowchart illustrations and/or block diagrams, can be implemented bycomputer program instructions. These computer program instructions maybe provided to a processor of a general purpose computer, specialpurpose computer, or other programmable data processing apparatus toproduce a machine, such that the instructions, which execute via theprocessor of the computer or other programmable data processingapparatus, create means for implementing the functions/acts specified inthe flowchart and/or block diagram block or blocks.

These computer program instructions may also be stored in a computerreadable medium that can direct a computer, other programmable dataprocessing apparatus, or other devices to function in a particularmanner, such that the instructions stored in the computer readablemedium produce an article of manufacture including instructions whichimplement the function/act specified in the flowchart and/or blockdiagram block or blocks.

The computer program instructions may also be loaded onto a computer,other programmable data processing apparatus, or other devices to causea series of operational steps to be performed on the computer, otherprogrammable apparatus or other devices to produce a computerimplemented process such that the instructions which execute on thecomputer or other programmable apparatus provide processes forimplementing the functions/acts specified in the flowchart and/or blockdiagram block or blocks.

The flowchart and block diagrams in the Figures illustrate thearchitecture, functionality, and operation of possible implementationsof systems, methods, and computer program products according to variousembodiments of the present invention. In this regard, each block in theflowchart or block diagrams may represent a module, segment, or portionof instructions, which comprises one or more executable instructions forimplementing the specified logical function(s). In some alternativeimplementations, the functions noted in the block may occur out of theorder noted in the figures. For example, two blocks shown in successionmay, in fact, be executed substantially concurrently, or the blocks maysometimes be executed in the reverse order, depending upon thefunctionality involved. It will also be noted that each block of theblock diagrams and/or flowchart illustration, and combinations of blocksin the block diagrams and/or flowchart illustration, can be implementedby special purpose hardware-based systems that perform the specifiedfunctions or acts or carry out combinations of special purpose hardwareand computer instructions.

References in the claims to an element in the singular is not intendedto mean “one and only” unless explicitly so stated, but rather “one ormore.” All structural and functional equivalents to the elements of theabove-described exemplary embodiment that are currently known or latercome to be known to those of ordinary skill in the art are intended tobe encompassed by the present claims. No claim element herein is to beconstrued under the provisions of 35 U.S.C. section 112, sixthparagraph, unless the element is expressly recited using the phrase“means for” or “step for.”

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention. Asused herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”and/or “comprising,” when used in this specification, specify thepresence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof.

The corresponding structures, materials, acts, and equivalents of allmeans or step plus function elements in the claims below are intended toinclude any structure, material, or act for performing the function incombination with other claimed elements as specifically claimed. Thedescription of the present invention has been presented for purposes ofillustration and description, but is not intended to be exhaustive orlimited to the invention in the form disclosed. Many modifications andvariations will be apparent to those of ordinary skill in the artwithout departing from the scope and spirit of the invention. Theembodiment was chosen and described in order to best explain theprinciples of the invention and the practical application, and to enableothers of ordinary skill in the art to understand the invention forvarious embodiments with various modifications as are suited to theparticular use contemplated.

What is claimed is:
 1. A method for retrospective snapshot creationcomprising: creating, by a processor, a first snapshot that captureslogical state of a data store at a first time in a time range, whereincreation of the first snapshot is based on: determining existence of asecond snapshot that captures logical state of the data store; andrecording a retrospective snapshot at a last valid log address offsetprior to the first time upon a determination that the second snapshotexists based on: determining at least one of: whether log addressoffsets from a first log entry of a log to a log entry of the log at thefirst time are contiguous; and whether log address offsets from thesecond snapshot to the first time are contiguous.
 2. The method of claim1, wherein log addresses are not reused, monotonically increase and arecontiguous.
 3. The method of claim 2, wherein: the second snapshot has alargest log address offset than other snapshots prior to the first time;and the time range is between a first date and a second date.
 4. Themethod of claim 3, wherein the second snapshot is a retrospectivesnapshot for a log address corresponding to a timestamp at the firsttime.
 5. The method of claim 4, wherein creation of the retrospectivesnapshot fails upon at least one of: the log address offsets from thefirst log entry to the log entry at the first time are non-contiguous;and the log address offsets from the second snapshot to the first timeare non-contiguous.
 6. The method of claim 1, further comprisingrecording a timestamp for each log entry in the log.
 7. The method ofclaim 1, further comprising locating a log address for a correspondingkey using a secondary index on a timestamp.
 8. A computer programproduct for retrospective snapshot creation, the computer programproduct comprising a computer readable storage medium having programinstructions embodied therewith, the program instructions executable bya processor to cause the processor to: create, by a processor, a firstsnapshot that captures logical state of a data store at a first time ina time range, wherein creation of the retrospective snapshot is basedon: determine, by the processor, existence of a second snapshot thatcaptures logical state of the data store; and record, by the processor,a retrospective snapshot at a last valid log address offset prior to thefirst time upon a determination that the second snapshot exists basedon: determining, by the processor, at least one of: whether log addressoffsets from a first log entry of a log to a log entry of the log at thefirst time are contiguous; and whether log address offsets from thesecond snapshot to the first time are contiguous.
 9. The computerprogram product of claim 8, wherein log addresses are not reused,monotonically increase and are contiguous.
 10. The computer programproduct of claim 9, wherein: the second snapshot has a largest logaddress offset than other snapshots prior to the first time; and thetime range is between a first date and a second date.
 11. The computerprogram product of claim 10, wherein the second snapshot is aretrospective snapshot for a log address corresponding to a timestamp atthe first time.
 12. The computer program product of claim 11, whereincreation of the retrospective snapshot fails upon at least one of: thelog address offsets from the first log entry to the log entry at thefirst time are non-contiguous; and the log address offsets from thesecond snapshot to the first time are non-contiguous.
 13. The computerprogram product of claim 12, further comprising program instructionsexecutable by the processor to cause the processor to: record, by theprocessor, a timestamp for each log entry in the log.
 14. The computerprogram product of claim 12, further comprising program instructionsexecutable by the processor to cause the processor to: locate, by theprocessor, a log address for a corresponding key using a secondary indexon a timestamp.
 15. An apparatus comprising: a memory storinginstructions; and a processor executing the instructions to create afirst snapshot that captures logical state of a data store at a firsttime in a time range, wherein creation of the first snapshot is based onthe processor: determining existence of a second snapshot that captureslogical state of the data store; and recording a retrospective snapshotat a last valid log address offset prior to the first time upon adetermination that the second snapshot exists based on the processorfurther: determining at least one of: whether log address offsets from afirst log entry of a log to a log entry of the log at the first time arecontiguous; and whether log address offsets from the second snapshot tothe first time are contiguous.
 16. The apparatus of claim 15, wherein:log addresses are not reused, monotonically increase and are contiguous;and the second snapshot has a largest log address offset than othersnapshots prior to the first time.
 17. The apparatus of claim 16,wherein: the time range is between a first date and a second date; andthe second snapshot is a retrospective snapshot for a log addresscorresponding to a timestamp at the first time.
 18. The apparatus ofclaim 17, wherein creation of the retrospective snapshot fails upon atleast one of: the log address offsets from the first log entry to thelog entry at the first time are non-contiguous; and the log addressoffsets from the second snapshot to the first time are non-contiguous.19. The apparatus of claim 15, wherein the processor further executesinstructions comprising recordation of a timestamp for each log entry inthe log.
 20. The apparatus of claim 15, wherein the processor furtherexecutes instructions comprising locating a log address for acorresponding key using a secondary index on a timestamp.